[quote]Or, it would be better to have only one IDE but allowing to develop in both versions?[/quote]I think it's best to just use Qt Creator 3.0 for both versions. You can add Qt 4.8 to Qt Creator 3.0:

Well, I finally decided to try to install Qt5. I stumbled into a nightmare of faulting dependencies, requests to uninstall some important (for my projects: PCL 1.6, VTK 5.8, and so on) libraries, and restoring my system to its previous state. 7 hours of work and stress.

[quote]I saw that some Qt5 components were not installed. Started Qt Creator, and it was still 2.5.2, not the newer 3.0. And it looked somewhat ugly.[/quote]Yes, distro packages will (almost) always lag behind.

I don't know what it's like now, but a few years ago Ubuntu changed Qt a lot in their packages, making it not properly compatible with other systems.

[quote]Would it have been better if I had downloaded Qt5 directly from Qt Project and installed it directly?[/quote]Yes. I recommend that. That's what I did, because I needed new features from Qt 5.2 for my app.

[quote]How would this work with my actual Qt4 installed from Ubuntu official repositories?[/quote]When you deploy your app, set LD_LIBRARY_PATH for your app. It will then ignore the Qt in your system.